1. Field of the Invention
The present invention relates to a cooling roll, ribbon-shaped magnetic materials, magnetic powders and bonded magnets. More specifically, the present invention relates to a cooling roll, a ribbon-shaped magnetic material manufactured by using the cooling roll, a magnetic powder formed from the ribbon-shaped magnetic material and a bonded magnet manufactured using the magnetic powder.
2. Description of the Prior Art
Rare-earth magnetic materials formed from alloys containing rare-earth elements have high magnetic properties. Therefore, when they are used for magnetic materials for motors, for example, the motors can exhibit high performance.
Such magnetic materials are manufactured by the quenching method using a melt spinning apparatus, for example. Hereinbelow, explanation will be made with regard to the manufacturing method using the melt spinning apparatus.
FIG. 23 is a sectional side view which shows the situation caused at or around a colliding section of a molten alloy with a cooling roll in the conventional melt spinning apparatus which manufactures a ribbon-shaped magnetic material by means of a single roll method.
As shown in this figure, in the conventional method, a magnetic material made of a predetermined alloy composition (hereinafter, referred to as xe2x80x9calloyxe2x80x9d) is melt and such a molten alloy 60 is injected from a nozzle (not shown in the drawing) so as to be collided with a circumferential surface 530 of a cooling roll 500 which is rotating relative to the nozzle in the direction indicated by the arrow A in FIG. 23. The alloy which is collided with the circumferential surface 530 is quenched (cooled) and then solidified, thereby producing a ribbon-shaped alloy in a continuous manner. This ribbon-shaped alloy is called as a melt spun ribbon. Since the melt spun ribbon was quenched in a rapid cooling rate, its microstructure has a structure composed of an amorphous phase or a microcrystalline phase, so that it can exhibit excellent magnetic properties as it is or by subjecting it to a heat treatment. In this regard, it is to be noted that the dotted line in FIG. 23 indicates a solidification interface 710 of the molten alloy 60.
The rare-earth elements are liable to oxidize. When they are oxidized, the magnetic properties thereof tend to be lowered. Therefore, normally, the manufacturing of the melt spun ribbon 80 is carried out under an inert gas atmosphere.
However, this causes the case that gas enters between the circumferential surface 530 and the puddle 70 of the molten alloy 60, which results in formation of dimples (depressions) 9 in the roll contact surface 810 of the melt spun ribbon 80 (that is, the surface of the melt spun ribbon which is in contact with the circumferential surface 530 of the cooling roll 500). This tendency becomes prominent as the peripheral velocity of the cooling roll 500 becomes large, and in such a case the area of the formed dimples becomes also larger.
In the case where such dimples 9 (especially, huge dimples) are formed, the molten alloy 60 can not sufficiently contact with the circumferential surface 530 of the cooling roll 500 at the locations of the dimples due to the existence of the entered gas, so that the cooling rate is lowered to prevent rapid solidification. As a result, at portions of the melt spun ribbon where such dimples are formed, the crystal grain size of the alloy becomes coarse, which results in lowered magnetic properties.
Magnetic powder obtained by milling such a melt spun ribbon having the portions of the lowered magnetic properties has larger dispersion or variation in its magnetic properties. Therefore, bonded magnets formed from such magnetic powder can have only poor magnetic properties, and corrosion resistance thereof is also low.
In view of the above problem involved in the prior art, it is an object of the present invention to provide a cooling roll which can manufacture a magnet having excellent magnetic properties and reliability, as well as a ribbon-shaped magnetic material manufactured using the cooling roll, a magnetic powder formed from the magnetic material and a bonded magnet formed from the magnetic powder
In order to achieve the above object, the present invention is directed to a cooling roll for manufacturing a ribbon-shaped magnetic material by colliding a molten alloy to a circumferential surface of the cooling roll so as to cool and then solidify it, wherein the circumferential surface of the cooling roll has dimple correcting means for dividing dimples to be produced on a roll contact surface of the ribbon-shaped magnetic material which is in contact with the circumferential surface of the cooling roll.
According to the above structure, it becomes possible to provide a cooling roll which can manufacture magnets having excellent magnetic properties and excellent reliability.
In this invention, it is preferred that the cooling roll includes a roll base and an outer surface layer provided on an outer peripheral portion of the roll base, and the outer surface layer has said dimple correcting means. This arrangement makes it possible to provide magnets having especially excellent magnetic properties.
In this case, it is preferred that the outer surface layer of the cooling roll is formed of a material having a heat conductivity lower than the heat conductivity of the structural material of the roll base at or around a room temperature. This makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, thereby enabling to provide magnets having especially excellent magnetic properties.
Further, the outer surface layer of the cooling roll is preferably formed of a ceramics. This also makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, thereby enabling to provide magnets having especially excellent magnetic properties. Further, the durability of the cooling roll is also improved.
Further, in the present invention, it is preferred that the outer surface layer of the cooling roll is formed of a material having a heat conductivity equal to or less than 80 Wxc2x7mxe2x88x921xc2x7Kxe2x88x921 at or around a room temperature. This also makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, so that it is possible to provide magnets having especially excellent magnetic properties.
Furthermore, it is also preferred that the outer surface layer of the cooling roll is formed of a material having a coefficient of thermal expansion in the range of 3.5xe2x88x9218[xc3x9710xe2x88x926Kxe2x88x921] at or around a room temperature. According to this, the surface layer is firmly secured to the base roll of the cooling roll, so that peeling off of the surface layer can be effectively prevented,
In the present invention, it is also preferred that the average thickness of the outer surface layer of the cooling roll is 0.5 to 50 xcexcm. This also makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, so that It is possible to provide magnets having especially excellent magnetic properties.
Moreover it is also preferred that the outer surface layer of the cooling roll is manufactured without experience of machining process. By using such a cooling roll, the surface roughness Ra of the circumferential surface of the cooling roll can be made small without machining process such as grinding or polishing.
In the present invention, it iIs preferred that the dimple correcting means includes at least one ridge formed on the circumferential surface of the cooling roll. This makes it possible to divide dimples to be produced on the roll contact surface effectively, so that it is possible to provide magnets having especially excellent magnetic properties.
In this case, it is preferred that the average width of the ridge is 0.5-95 xcexcm. This makes it possible to divide dimples to be produced on the roll contact surface more effectively, so that it is possible to provide magnets having especially excellent magnetic properties.
Further, it is also preferred that the ridge is provided by forming at least one groove in the circumferential surface of the cooling roll. By forming the ridge in this way, it becomes possible to adjust the width of the ridge and the like accurately.
Furthermore, it is also preferred that the average width of each groove is 0.5-90 xcexcm. This also makes it possible to divide dimples to be produced on the roll contact surface more effectively, so that it is possible to provide magnets having especially excellent magnetic properties.
Furthermore, it is also preferred that the average height of the ridge or the average depth of the groove is 0.5-20 xcexcm. This also makes it possible to divide dimples to be produced on the roll contact surface more effectively, so that it is possible to provide magnets having especially excellent magnetic properties.
Moreover, it is also preferred that the ridge or groove is formed spirally with respect to the rotation axis of the cooling roll. According to such a structure, it is possible to form the cooling roll with the grooves and ridges relatively easily. Further, this also makes it possible to divide dimples to be produced on the roll contact surface more effectively, so that it is possible to provide magnets having especially excellent magnetic properties.
Moreover, it is also preferred that the at least one ridge or groove includes a plurality of ridges or grooves which are arranged in parallel with each other through an average pitch of 0.5-10 xcexcm. According to this arrangement of the ridges or grooves, it is possible to make dispersion or variation in the cooling rates at various portions of the molten alloy small, so that it is possible to provide magnets having especially excellent magnetic properties.
Further, in the present invention, it is also preferred that the ratio of the projected area of the ridge or groove with respect to the projected area of the circumferential surface is equal to or greater than 10%. This makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, so that it is possible to provide magnets having especially excellent magnetic properties.
Another aspect of the present invention is directed to a ribbon-shaped magnetic material which is manufactured by colliding a molten alloy to a circumferential surface of a cooling roll so as to cool and then solidify it, wherein the circumferential surface of the cooling roll has dimple correcting means f or dividing dimples to be produced on a roll contact surface of the ribbon-shaped magnetic material which is in contact with the circumferential surface of the cooling roll.
According to the above structure, it is possible to provide a ribbon-shaped magnetic material which can provide magnets having especially excellent magnetic properties and having excellent reliability.
In this case, it is preferred that the cooling roll Is provided with the dimple correcting means described above on the circumferential surface thereof. This makes It possible to provide magnets having especially excellent magnetic properties.
In this ribbon-shaped magnetic material it is preferred that a roll contact surface of the ribbon-shaped magnetic material is formed with grooves or ridges so that dimples formed on the roll contact surface thereof are divided by the grooves or ridges. This also makes it possible to provide magnet shaving especially excellent magnetic properties.
Further, in this ribbon-shaped magnetic material, it is also preferred that the dimples produced on the roll contact surface of the ribbon-shaped magnetic material upon solidification thereof include huge dimples each having an area equal to or greater than 2000 xcexcm2, in which the ratio of the area in the roll contact surface occupied by thus produced huge dimples with respect to the total area of the roll contact surface of the ribbon-shaped magnetic material is equal to or less than 10%. Such ribbon-shaped magnetic material has less dispersion in crystal grain sizes at various portions thereof, so that it is possible to provide magnets having especially excellent magnetic properties.
Furthermore, in the ribbon shaped magnetic material, it is also preferred that the division of the dimples to be produced is carried out by transferring the shape of at least a part of the circumferential surface of the cooling roll to the roll contact surface of the ribbon-shaped magnetic material. This also makes it possible to make the dispersion in the crystal grain sizes at the various portions of the ribbon-shaped magnetic material small, so that it is possible to provide magnets having especially excellent magnetic properties.
In this case, it is preferred that the average thickness of the ribbon-shaped magnetic material is 8-50 xcexcm. By using such a ribbon-shaped magnetic material, it is possible to provide magnets having more excellent magnetic properties.
Other aspect of the present invention is directed to a magnetic powder which is obtained by milling the ribbon-shaped magnetic material which is manufactured by colliding a molten alloy to a circumferential surface of a cooling roll so as to cool and then solidify it, wherein the circumferential surface of the cooling roll has dimple correcting means for dividing dimples to be produced on a roll contact surface of the ribbon-shaped magnetic material which is in contact with the circumferential surface of the cooling roll.
By using such a magnetic powder, it is possible to provide magnets having excellent magnetic properties and reliability.
In this case, it is preferred that the magnetic powder is subjected to at least one heat treatment during or after the manufacturing process thereof. This makes it possible to provide magnets having more excellent magnetic properties.
Further, it is also preferred that the mean particle size of the magnetic powder lies within the range of 1-300 xcexcm. This also makes it possible to provide magnets having more excellent magnetic properties.
Furthermore, it is also preferred that the magnetic powder has a composite structure which is composed of a hard magnetic phase and a soft magnetic phase. This also makes it possible to provide magnets having especially excellent magnetic properties.
In this case, it is preferred that the average crystal grain size of each of the hard magnetic phase and the soft magnetic phase is 1-100 nm. This also makes it possible to provide magnets having excellent magnetic properties, especially excellent coercive force and rectangularity.
The other aspect of the present invention is directed to a bonded magnet which is manufactured by binding the magnetic powder which is obtained by milling the ribbon-shaped magnetic material which is manufactured by colliding a molten alloy to a circumferential surface of a cooling roll so as to cool and then solidify it, wherein the circumferential surface of the cooling roll has dimple correcting means for dividing dimples to be produced on a roll contact surface of the ribbon-shaped magnetic material which is in contact with the circumferential surface of the cooling roll.
The bonded magnet manufactured as described above can have especially excellent magnetic properties and reliability.
In this case, it is preferred that the intrinsic coercive force (HCJ) of the bonded magnet at a room temperature is in the range of 320-1200 kA/m. This makes it possible to provide a bonded magnet having excellent heat resistance and magnetizability as well as sufficient magnetic flux density.
In this case, it is preferred that the maximum magnetic energy product (BH)max of the bonded magnet is equal to or greater than 40 kJ/m3. By using such a bonded magnet, it is possible to provide high performance small size motors.